Studying the Low Frequency Noise
of Tunneling Sensors

One of the issues affecting the use of tunneling transducers in high performance sensors is the variation of the control voltage over time. In data collected at JPL, it was observed that the operating voltage was dominated by a sinusoid with a period of one day. Initial experiments conducted at Stanford have shown a similar variation tracking the laboratory air-conditioning system limit cycle. Further experiments have verified the relation between ambient temperature and variation in the tunneling sensor control voltage. This variation was initially attributed to thermal expansion of the epoxy bond between the wafers in the transducer[1], but experiments have indicated that differential thermal expansion in the membrane was responsible[2].

Having identified the dominant source of low frequency noise, we have begun the process of minimizing its impact by modifying the device structure. Our recent efforts have concentrated on two new transducer designs which have modified membrane structures. These new transducers feature noise and sensitivity characteristics which are up to 100 times better than tunneling transducers developed in previous experiments. As such these results represent a significant improvement in the state of the art for stability of tunneling displacement transducers.

Previous tunneling transducers, shown in figure 1a, have featured a 2 mm diameter, gold coated, nitride membrane which is electrostatically deflected toward a blunt gold coated tunneling tip. Unfortunately, the difference in expansion coefficient between the gold and the nitride is sufficient to produce a deflection at the tip with small changes in ambient temperature. The new designs attack this problem by either balancing the bimorph or reducing its length: one transducer variation, shown in figure 1b, includes an additional gold film on the upper surface of the membrane, the other variation, shown in figure 1c, has the gold on the underside of the membrane patterned as shown in figure 2. In the latter design, gold remains on the membrane only above the tip and in thin concentric rings. This allows for tunneling and electrostatic deflection, but reduces the length of the bimorph significantly.

When the output voltage of the three tunneling transducers is observed over a period of several minutes in the absence of an input signal, the transducers all follow the changes in ambient temperature, but the amount of drift varies with each design. Figure 3 shows force noise spectra associated with these devices. Since the membrane stiffness varies between transducer designs, force noise allows for a fairer comparison between different device designs than displacement noise. The noise spectra in figure 3 show an improvement of over an order of magnitude from traditional membranes to gold sandwich membranes and an additional improvement to patterned gold devices.

These promising results indicate that the noise floor in tunneling transducers may be significantly reduced by making minor changes in the transducer structure. The main noise sources in these devices are still due to the transducer structure and do not reflect fundamental tunneling noise. This work represents a significant step forward in the optimization of the membrane transducer structure for low noise operation at low-frequencies.